Drug therapy for medical disorders often requires improvement in aspects such as pharmacokinetics, efficacy, or adverse effects. For example, many anti-cancer drugs have a very narrow therapeutic window, i.e., a small difference between the drug levels required to exert a beneficial anti-tumor effect, and those causing adverse effects, some of which may be dose-limiting or life-risking. As another example, it is often desirable for many drugs used in various fields, such as anti-cancer drugs, anti-inflammatory drugs, or drugs used to treat neurological or psychiatric disorders, to be administered as inactive pro-drugs, with a subsequent slow, sustained release of the active drug, thereby enabling prolonged drug circulation time and prolonged drug activity at its target sites. Furthermore, it is often desirable for a drug, to be targeted to a focus of disease, i.e., to have, following its administration to the patient, a concentration gradient between target and non-target tissues, with relatively high levels of the pro-drug and/or the active drug at the foci of disease, compared to non-target tissues.